A cooperative study was conducted from 1958 to 1965. Its purpose was to collect data on patients with subarachnoid hemorrhage and intracranial aneurysms. The results were analyzed and published. However, these analyses did not answer questions regarding the long-term prognosis with and without treatment. Therefore, the present study was designed to follow-up those patients in order to answer the following questions: 1) What is the prognosis for rebleeding and survival for patients with a ruptured aneurysm if the aneurysm has not been treated surgically? 2) What is the prognosis for bleeding from a proven unruptured aneurysm? 3) What is the prognosis for rebleeding and survival if an aneurysm has been treated by carotid ligation? 4) What is the risk of late cerebral ischemic complications resulting from surgical carotid occlusion for aneurysms? 5) What is the natural history of untreated arteriovenous malformations? 6) If subarachnoid hemorrhage has occurred and adequate angiographic studies fail to delineate a cause, what are the risks of rebleeding and what is its cause if discovered?